In carrying out input scanning using a photosensor array (for instance, a CCD photosensor array or an array of photodiodes), voltage drift in the photosensor array output caused by characteristic variations in each sensor element thereof ("calibration") must be corrected.
On the other hand, when an array of photodiodes is used as a device for analyzing a laser beam modulated with the image of an original by a swinging galvano mirror or a revolving polygon mirror, the quantity of the laser beam captured by each element of the photosensor array is statically unequal. This is due to variations in the velocity of the laser beam, aberrations in the lens employed and fluctuations in the laser beam itself. Also, in scanning an original by projecting the image thereof on a CCD photosensor array, the lenses or light sources employed create errors in the amount of light transmitted, which must be corrected (this is called "shading correction").
Calibration and shading correction have conventionally been carried out by storing data for correcting a distortion factor of each element of a photosensor array, and then carrying out a corrective computation for image data obtained by the photosensor array using corresponding correction data. In this connection, Japanese Patent Laid Open No. 57-119565 discloses a method in which correction values corresponding to all the elements of a photosensor array are prestored in a memory. Japanese Patent Laid Open No. 58-145273 discloses a method by which gain correction data corresponding to all the elements of a photosensor array are prestored in a memory, and Japanese Patent Laid Open No. 58-19187 discloses a method in which the inverse values of correction values instead of the correction values themselves are prestored in a memory to provide a simple correction work. However, in each of the above methods, some correction data must be obtained; this is troublesome, and moreover, as regards the last mentioned reference, inverse correction is not precise.
In addition, there is disclosed in Japanese Patent Laid Open No. 58-27466 a method in which correction values corresponding to particular readout positions of a photosensor array are stored. Values read from the other positions of the array are obtained through interpolation, requiring circuitry and programming.
Japanese Patent Laid Open No. 58-131862 discloses a method in which an input scanning laser beam is modulated by data obtained from a light quantity detection sensor provided beside a photosensor array to maintain the incidence light quantity of the laser beam on the photosensor array constant. Because this method necessitates a light quantity detection sensor which is required to monitor all the scanning points as equally as possible, the method is impractical. U.S. patent application Ser. No. 671,984 assigned to the assignee of this invention discloses a method in which a shading phenomenon is corrected by multiplying image data obtained by a photosensor, by a correction signal output from a low-pass filter of a phase lock loop circuit in conformity with the variation of the velocity of a scanning beam on an original. This method is also effective in compensating for a shading phenomenon but it cannot carry out calibration simultaneously.